Wet rub tester



F. R. ADAMS WET RUB TESTER June 26, 1962 Filed April 7, 1960 INV EN TOR.

foeearr 840 4044/:

United States Patent 3,040,559 WET RUB TESTER Forrest Ray Adams, Glens Falls, N.Y., assignor to International Paper Company, New York, N.Y., a corporation of New York I Filed Apr. 7, 1960, Ser. No. 20,653 2 Claims. (Cl. 73-7) This invention relates to paper testing. More particularly, it relates to apparatus capable of measuring the resistance of coated paper to wet rub and to the method of testing wet rub employing such apparatus. 7

The wet rub resistance of coated paper is its ability to resist or withstand destruction by a combination of conditions, i.e., moisture and abrasion. Consequently, eflicicnt equipment and reliable methods by which to measure such resistance are highly desirable.

In the past, the wet rub resistance property has been measured by placing a spot of water on a sheet of the coated paper, puddling the spot with a finger, and observing the result. A variation of this method calls for placing a spot of water on a sheet of coated paper, placing a finger in the spot of water, and drawmg the finger across the sheet onto a piece of black paper positioned under and to one side of the sheet a specified number of times. The black papers are then dried and the relative brightness of the spot of coating removed is taken as the measure of the wet rub resistance. Again, the test disclosed in Tappi Monograph No. 17, Starch and Starch Products in Paper Coating, New York, 1957, p. 96, contemplates a drop or two of water applied to the coated paper and the rubbing of such coated paper on a black flint surface with a minimum pressure of an index finger. The wet rub index is the number of rubs to give the first appearance of white coating transferred to the black flint surface. The sources of variation and error in such methods are manifest and, as a result, workers in the field have been led to produce a mechanical, recipromting arm type of wet rub tester. However, even this device suffers in effectiveness from the fact that it relies heavily on the subjective reaction of the observer or operator to the phenomenon taking place, since he, not the apparatus, determines the particular moment at which the paper coating breaks.

Accordingly, it is an object of the present invention to reduce the factors of human inconstancy and error in the measurement of wet rub resistance and to make such measurement more automatic. It is a further object to afford means by which standards of wet rub resistance measurement can be established. Again, it is an object of the present invention to provide an apparatus and a pended for substantially free rotation fiom the idler raising and lowering means and in the journal means; the idler raising and lowering means are connected and vertically movable with respect to the fixed support means; the light roll has an uppermost portion and a lowermost portion, is fixedly disposed below the idler for contact therewith along the uppermost portion when the idler raising and lowering means are in the lowered position, and is suspended for rotation from the base means and in the bearing means; the container is fixedly disposed below the lowermost portion of the roll for contact therewith by the water; the drive speed control means are connected to the drive means. And, in combination, these parts operate together so that, when pieces or strips of paper are mounted on the idler and it is placed in the lowered position, theroll being driven by the drive means turns tangentially to the idler and rotates it, while moistening the paper with water picked up from the container and abrading the paper. It will be readily seen that, because the idler is heavy and the roll is light, such abrasion is caused not only by the rolling friction between the two, but by the inevitable sliding friction between the two. Alternatively, a partially braked idler can be used in place of a substantially freely rotatable heavy idler.

More particularly and in one embodiment of the apparatus of the invention, the idler is a wheel having a rim and a centrally disposed axle suspended by its ends for substantially free rotation about its horizontally disposed longitudinal axis. Preferably, the rim is annular and smoothly surfaced or even highly polished, since it is to such surface that the pieces or strips of paper to be tested are mounted. Then, the idler wheel raising and lowering means are two parallel lever arms each having on one end journal means in receipt of an end of the axle of the idler wheel and being on the other end connected and vertically rotatable with respect to the fixed support means. The roll is a cylinder, preferably of rubber, having uppermost and lowermost portions, is fixedly disposed below the idler wheel rim for contact therewith along the uppermost portion when the idler wheel raising and lowering lever arms are in the lowered position, and has a centrally disposed shaft suspended method of employing such apparatus which simulate the near its ends for rotation about its horizontally disposed longitudinal axis. Any roll comparable to rubber in surface and resiliency properties when employed in offset printing press transfer rolls or blankets can also be used. The base has ball bearings which are in receipt of the ends of the shaft, the shaft is connected to an electric motor which supplies the roll drive, and a variable speed control such as a commercially available Variac and a switch are electrically connected to the motor.

For a better understanding of the invention, reference should be had to the attached FIGURE, which depicts an isometric perspective of the apparatus involved.

The figure shows idler wheel 1, including axle 2 and rim 3, suspended from parallel idler wheel raising and lowering lever arms 4a and 4b and in journal 5. Rim 3 is covered about its circumference by test paper sample strip 6. Lever arms 4a and 4b are connected and vertically rotatable with respect to fixed support 7 at pivot 8 and are held in the raised position .by means of removable pin 9 and hole 10 sized therefor in fixed support 7,

rubber-covered roll 11 is positioned below idler wheel rim 3, and shaft 12 of roll 11 is held in bearings 13a and 13b mounted in base 14. Water container 15 is positioned below roll 11. Further, shaft 12 is connected by means of coupling 16 and gear reduction box 17 to electric motor 18. The motor, in turn, is linked by electrical connections (not shown) to speed control box 19 and switch 20.

The test method contemplated by the present invention involves a first step of mounting sample pieces or strips of the paper to be tested on the idler. A preferred way of doing this is to place a strip of double-sided pressure sensitive tape, such as commercially available Scotch Brand tape, entirely around the rim of the idler and, next, to cover the exposed pressure sensitive tape strip face with a strip of the test paper sample originally cut longer than the circumference of the rim and then trimmed to give a substantially unbroken or continuous band of the sample around the rim. This mounting, of course, is done when the idler raising and lowering means 'are in a raised position.

The second step of the method of the invention is that of rotating the roll at a predetermined speed, i.e., starting and regulating the speed of the drive means, thereby causing a continuous changing of the lowermost portion of the roll and a uniform pick-up by such portion of the roll of moisture from the water container. The third step is one of placing the idler raising and lowering means in the lowered position, thereby bringing the test paper sample covered idler into contact with the uppermost portion of the rotating roll, at the beginning of a timed interval and the fourth step is one of returning the idler raising and lowering means to the raised position at the end of the timed interval. Both of these steps can be performed automatically, as well as manually. Then, for the fifth step of the method, the weight of the solids removed from the test paper sample strip by the application of moisture and abrasion thereto is determined. This determination is made by any one of a number of familiar techniques. The water in the container is decanted into a previously tared weighing dish or crucible or on and through filter paper, it is evaporated to dryness, and the solids collecting 'agent is reweighed to establish the weight of coating removed from the test paper sample. This increase can vary from zerofor a waterproof paper sample such as label paper-to a figure equalling the total coating weight of the test sample-for certain types of clay-starch coated papers used in letterpress printing applications.

For a clearer understanding of the method of the invention and its eifectiveness, reference should be made to the following examples and tables:

EXAMPLE I Using a testing machine substantially like that shown in the attached figure, a test was run to show the relationship between the coating removed and the adhesive base of samples of various commercially available papers. In the machine, the idler was of steel, had a radius of 3 inches, and weighed 539.5 grams, while the rubber cylin der had an outside radius of 1.5 inches and an inside radius of about 0.5 inch. The cylinder had a Shore durometer hardness of 67. The water container held 30 ml. of distilled water (pH 5.5 to 6.0 at 72 R). A fresh strip of double-sided cellulose pressure sensitive tape (Scotch Brand #665) was placed entirely around the face of the rim of the upraisecl idler wheel for each sample and, then, the sample, cut wide and of a length greater than the circumference of the idler, was mounted on the tape The excess of the sample was trimmed off, so that a substantially unbroken band of paper extended over the face of the rim of the idler. The motor was thereafter turned on and, by means of the motor speed control, was set to turn the roll at 208 r.p.m., so as to give the idler a calculated speed of 105.0 r.p.m. Next, the test paper sample strip-covered idler was lowered against the uppermost portion of the rotating roll and was observed to rotate at about 90 r.p.m. for the 10 second interval of each test run, indicating a certain amount of slippage between the two rotating bodies. At the end of the second period, the idler was raised, the water in the container was decanted into a weighing dish and evaporated to dryness. The weight of the coating removed and the adhesive bases of the samples are shown in Table I as follows:

N ore-The adhesives here included enzyme converted pearl starch and butadiene-styrene co-polymer latex (commercially available Dow 512-18.).

EXAMPLE H 4 sheets of 70 lb. starch-latex offset paper were chosen at random and, from each, 2 strips were cut one inch from one edge and 2 strips were cut from the center. These strips were identified and then mixed and chosen at random for testing and were tested, in order to learn the variations in wet rub resistance between felt and wire sides and 'between sheets, using the same apparatus and technique as described in Example I. The results, to the nearest 0.01 gm., are shown in Table 11 as follows:

Table II Edge strips Center strips Sheet Avg,

row Felt Wire Felt Wire 1 1 1 2 1. 25 2 2 l 2 l. 75 1 2 2 1 1. 5O 2 2 l 1 1. 50

Avg. 001---" l. 5 1.75 1 25 1. 5 1 1.50

1 Avg. total series. Nora-Avg. total felt 1.38. Avg. total wire 1.63. Avg. total edge 1.63. Avg. total center 1.38.

EXAMPLE III 3 sheets of starch-latex ofiset paper were chosen at random from each of a number of stocks of different basis and production dates and 2 strips were cut from each sheet. These strips were then mixed and chosen at random for testing and were tested, in order to learn the variations in wet rub resistance between the various stocks,

using the same apparatus and technique as described in Example I. The results, to the nearest 0.01 gm., are shown in Table III as follows:

a heavy test sample idler, idler raising and lowering means, journal means, fixed support means, a light roll,

Table III Set 1 Set 2 Basis Avg. Av Prod. date weight Sheet Sheet total b Set Set row weight avg. avg. 1 2 3 1 2 3 45 1 1 0. 7 0 1 0 0.3 0. 5 0. 5 50 1 1 1 1. 0 2 1 2 1. 7 1. 3 1. 3 55 1 0 0 0.3 0 0 0 O 0. 2 55 0 1 1 0. 7 1 1 1 1. 0 0. 8 0. 6 55 0 1 1 0. 7 1 1 1 1. 0 0. 8 60 1 l 0 0. 7 1 0 1 0. 7 O. 7 0. 7 70 1 1 0 0. 7 0 1 1 O. 7 0. 7 70 2 2 1 1. 7 1 1 1 1. 0 1. 3 1. 3 70 2 1 2 1. 7 2 2 2 2. 0 1. 8

1 Avg. total series. No-rn.-Avg. total set: Set 1 equals 0.9, set 2 equals 0.9.

EXAMPLE IV Using the apparatus and technique described in Example I, a number of commercially available 60 1b. and 70 lb. papers were wet rub tested. The results are shown in Table 4 as follows:

1 These coatings separated from raw stock in large flakes.

By way of conclusions, it has been found that the papers tested fall into one of three, well-defined categories. The first category covers waterproof sheets of paper which lose no coating in the sample water. This category is characterized by the use of casein, Alpha protein, or relatively large amounts of the synthetic latices as the adhe sive. The second category covers those papers designed for use with offset printing presses characterized by the use of starch-latex combinations as the adhesives. The test apparatus and method of the invention will remove from 1 to 35 milligrams of coating and, in the majority of tests, below milligrams. Starch-based papers made for letterpress printing which are in the third category will give up from 50 to 80 milligrams of coating per test paper sample strip, depending on the weight of the coating on the sample.

It has also been found, in relation to the effectiveness of the apparatus and method of the invention that readings for test paper sample strips cut from the same sheet are normally within very close limits, while the variations between the readings for strips cut from difierent sheets run higher. The greatest variations are found to exist for papers of a particular basis weight of a grade made on different production days.

' What is claimed is:

1. A wet rub resistance testing apparatus comprising base means, bearing means, a container of water, drive means, and drive speed control means wherein the heavy test sample idler is suspended for substantially free rotation from the idler raising and lowering means and in the journal means; the idler raising and lowering means are connected and vertically movable with respect to the fixed support means; the light roll has an uppermost portion and a lowermost portion, is fixedly disposed below the idler for contact therewith along the uppermost portion when the idler raising and lowering means are in the lowered position, and is suspended for rotation from the base means and in the bearing means; the container is fixedly disposed below the lowermost portion of the roll for contact therewith by the water; the drive means are connected to the roll; and, the drive speed control means are connected to the drive means 2. A wet rub resistance testing apparatus consisting essentially of a heavy test sample wheel, two parallel wheel raising and lowering lever arms, journal means, fixed support means, a rubber cylinder, base means, ball bearings, a container of water, an electric motor, and drive speed control means wherein the heavy test sample wheel has an annular rim and a centrally disposed axle suspended by its ends for substantially free rotation about its horizontally disposed longitudinal axis from the two lever arms and in the journal means; the two lever arms each have on one end the journal means in receipt of one end of the axle and are on the other end connected and vertically rotatable with respect to the fixed support means; the-rubber cylinder has an uppermost portion and a lowermost portion, is fixedly disposed below the rim for contact therewith along the uppermost portion when the lever arms are in the lowered position, and has a centrally disposed shaft suspended near its ends for rotation about its horizontally disposed longitudinal axis from the base means and in the ball bearings; the container is fixedly disposed below the lowermost portion of the cylinder for contact therewith by the water; the electric motor is connected to the shaft; and, the drive speed control means are electrically connected to the motor.

References Cited in the file of this patent UNITED STATES PATENTS 1,444,803 Ratner et al Feb. 13, 1923 1,899,774 Rothchild et al. Feb. 28, 1933 2,101,322 Reed Dec. 7, 1937 2,367,838 Allen Jan. 23, 1945 2,561,133 Petkewicz July 17, 1951 2,687,641 Stout Aug. 31, 1954 

